I once sat hours over a bug I couldn't understand until I finally saw the '!' in the condition. I started to use triple '!!!' for a while to avoid overseeing them.

If (!!! enabled) {// like really really not enabled
enable() ;
}

After I read clean code I talked to a fellow developer about some concepts. Later I reviewed some code of him and he clearly got the concept (not)

Java
...
If (isTrue(someValue))...

public boolean isTrue(boolean value){
if(value == true)
return true;
else
return false;
}

One of my preferred functions is Collections.unmodifiableList(List).

What a relief was the introduction of the collection FW. And the Function above changed the usage of lists (also sets, maps) a lot. You could now just expose your internal list without worrying that somebody messes with the data.